It is known to use linear actuators in the most varied fields in the art, for moving mechanical parts, members of machines, or devices connected to a mobile part of the actuators themselves.
It is also known that linear actuators are used to apply forces or pressures on products, for example on semi-worked products to be deformed or shaped, or parts of machines, for example with the purpose of supporting or lifting weights of said machines.
Linear actuators with a piston are known, comprising a cylinder provided with a cylindrical tubular container, or liner, inside which a gaseous work fluid is contained, for example air, nitrogen or other gas, or liquid, or viscose substance for example oil. A plunger slides inside the liner; connected to a shaft, the plunger defines the piston which receives and transmits the thrust of the work fluid, or compresses and takes in the fluid itself.
Normally the cylinder also comprises a head and a bottom which close the two ends of the liner in a sealed manner.
For the operations that need high pressures and/or that require high forces to be applied, it is known to use oil as the work fluid.
Here and hereafter in the description, and in the claims, for ease of exposition we shall refer to piston type linear actuators by the term “actuators” and the corresponding singular form “actuator”, and linear actuators with a piston of the hydraulic type by the simplified expression “hydraulic actuators” and the corresponding singular form “hydraulic actuator”.
One of the requirements of actuators, especially in the case of hydraulic actuators, is that they must resist mechanical stresses due to the pressure inside the liner and directed both longitudinally along the longitudinal axis thereof and also radially in a direction perpendicular to said axis.
Known solutions for opposing radial stresses provide to use high-resistance materials studied for the purpose, to thicken the perimeter walls of the liner and/or to coat the perimeter walls with reinforcement materials, for example composites.
Solutions are also known to counter-balance the axial stresses, which provide to use metal tie rods positioned externally to the liner, parallel to the longitudinal axis thereof, and attached to the head and the bottom, normally by screwing.
Another requirement is to contain the mass of the components of the actuators, to obtain advantages in performance, especially in terms of weight/power ratio.
In various applications, for example in pumps for concrete, the reduction in mass and therefore in weight of the components is a sensitive subject in design, because the performance, in terms of weights and sizes, of the whole machine largely depends on the mass of the distribution arm.
It must also be considered that a truck-mounted pump comprises a plurality of segments that clamp the distribution arm, normally from four to seven, with corresponding movement actuators, and that such machines need to have longer and longer arms, in order to reach long distances.
Design efforts have been concentrated on reducing the overall weight of the distribution arm, while keeping the same length. A further requirement has therefore arisen, that is, to also reduce the weight of the actuators associated with the segments, while keeping the same obtainable power.
Normally, in this application, the heavier the distribution arm, the greater the overall sizes of the machine are needed to discharge and absorb the working loads.
In documents EP0464202A1 and U.S. Pat. No. 5,198,761A a pressurized receptacle is described, made of composite material and comprising a series of elements to be assembled using fibrous material wound in several passes and crossed over around an internal cylinder.
Document GB2203215A describes a double-effect cylinder provided with support tie rods against stresses.
One disadvantage of known actuators, in particular those provided with longitudinal metal tie rods, is that they are heavy and bulky, at least in a radial direction, because of the mass and sizes of the tie rods themselves, which must be suitably sized so as to resist the working stresses. Therefore, this disadvantage is more serious as the working stresses increase, as happens in the case of hydraulic actuators.
Another disadvantage of known actuators is that the metal tie rods not only make the actuators themselves heavier, but are also more subject to damage due to knocks or impacts that can occur during use.
Another disadvantage is that often known cylinders are assembled in several parts, making the final structure even heavier and more complex, given the large number of components and the operations needed to assemble them.
One purpose of the present invention is to obtain a cylinder at least partly made of composite material, usable as part of an actuator and, with the same working mechanical stresses supported, able to allow a reduction in weight of the actuator compared with known actuators.
Another purpose of the present invention is to obtain a cylinder that, with the same working mechanical stresses supported, is able to contain the sizes, at least in a radial direction, of the actuator of which it is part.
Another purpose of the present invention is to obtain a cylinder in which the tie rods are advantageously disposed in a substantially longitudinal direction and distanced from each other, so as to obtain an efficient resistance to axial stresses and to reduce the overall weight of the structure.
Another purpose of the present invention is to obtain a cylinder in which the components and parts to be assembled are reduced to a minimum, or at least limited compared with the parts that make up known cylinders.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.